Coated optical fibers

ABSTRACT

The invention relates to coated optical fibers comprising soft primary coatings and to such primary coatings for protecting glass optical fibers having a sufficient high resistance against cavitation. In particular, the primary coatings have a cavitation strength at which a tenth cavitation appears (σ 10   cav ) of at least about 1.0 MPa as measured at a deformation rate of 0.20% min −1  and of at least about 1.4 times their storage modulus at 23° C. The coating preferably shows strain hardening in a relative Mooney plot, preferably has a strain energy release rate Go of about 20 J/m 2  or more, and preferably has a low volumetric thermal expansion coefficient. The invention furthermore provides a method and apparatus for measuring the cavitation strength of a primary coating.

This application is a divisional of application Ser. No. 11/269,765,filed Nov. 9, 2005, now U.S. Pat. No. 7,706,659; which is a continuationof application Ser. No. 09/989,703, filed Nov. 21, 2001, now U.S. Pat.No. 7,067,564; which is a continuation-in-part of application Ser. No.09/717,377, filed Nov. 22, 2000 now abandoned, the entire contents ofwhich is hereby incorporated by reference in this application.

FIELD OF THE INVENTION

The present invention relates to a coated optical fiber comprising aprimary and secondary coating, to a radiation curable primary coatingcomposition, to a combination of a primary and secondary coating, and toa ribbon comprising at least one of said coated optical fibers and to amethod and apparatus for measuring cavitation strength of a coating foruse as a primary coating on an optical fiber.

DESCRIPTION OF RELATED ART

Because optical fibers are fragile and easily broken, the optical fibersare usually coated with a coating material which is a radiation curableresin composition. The transmission characteristics of optical fibersare known to be significantly affected by properties such as modulus orthe like of the primary coating material which is in direct contact withthe optical fibers. When optical fibers are coated with a primarycoating material having an equilibrium modulus of about 2 MPa or higher,the transmission loss of the optical fibers may increase because ofdecreased buffering effect. A material having a low modulus ofelasticity is, therefore, desirable as the primary coating material.Primary coating materials having an equilibrium modulus of 1.5 MPa orless are thus of interest as is described for example by Bouten et al.(J. of Lightwave Technology, Vol. 7 April 1989, p. 680-686).

There is a long felt need in the optical fiber industry to use suchsofter (lower modulus) primary coatings to introduce a higher resistanceagainst microbending and thus to prevent attenuation losses. However,when using such low modulus primary coatings, and in particular, whenusing primary coatings having a modulus below 1.3 MPa, the strength ofthe coating is decreased and thus the integrity of the coating is atrisk. Hence, such coatings tend to be very fragile and can result in theformation of defects in the coating during processing or use of thecoated optical fiber.

It is described in WO99/08975 to prepare primary coatings having a lowsecant modulus (<1.5 MPa) while having a high tensile strength at break(>1.5 MPa) which are said to protect an optical fiber for a long periodof time in a safe and stable manner while obtaining an excellenttransmission performance.

However, these coatings need further improvement in strength orintegrity because defects still appear during the use of the coatedoptical fiber, in particular, under the influence of high stresses andtemperature extremes which the coated fiber has to withstand over time(during production, cabling or when buried under the ground). Thisproblem is further enhanced nowadays due to the increasing line speedsfor fiber drawing causing steeper cooling profiles, and allowing lesstime for relaxation.

It has now been found that a primary coating having an equilibriummodulus of about 1.5 MPa or lower, when coated on a glass optical fiberand when subsequently having a secondary coating (having a much higherTg) applied thereon, undergoes at least the following stress: when thetemperature decreases during the production process the secondarycoating passes its glass temperature (Tg) and enters the glassy statewhile the primary coating is still above it's glass temperature. Theprimary coating still intends to shrink when the temperature decreasesfurther, but is captured between the rigid secondary on the one hand andthe rigid glass substrate on the other hand. This precludes theshrinking process of the primary coating substantially. This stress canresult in loosening of the primary coating from the glass surface if theadhesion is insufficient (as is described by King and Aloisio in J.Electronic Packaging, June 1997, Vol. 119 p. 133-137 in an articletitled “Thermomechnical Mechanism for Delamination of Polymer Coatingsfrom Optical Fibers”). During coloring, cabling and possibly in thefield, the fibers may be cycled through high and low temperatures,causing comparable stress on the primary coating.

This stress has now been proven to also result in the appearance ofdefects within the coating. These defects are in fact ruptures withinthe primary coating itself which have to be regarded as distinct fromdelaminations at the interface of primary coating and glass. For thepurposes of the present invention, such defects in the coating arefurther called cavitations or cavities.

OBJECT OF THE INVENTION

It is an object of the present invention to obtain an optical fibercoated with a primary and secondary coating, of which the primarycoating has a sufficient high cavitation strength while having a lowmodulus.

Further, it is an object of the present invention to obtain soft primarycoatings with an equilibrium modulus of about 1.5 MPa or less havingsufficient resistance to cavitation to remain substantially free ofcavitations.

It is another object of the invention to provide a method and anapparatus to measure the cavitation strength.

It is a further object of the present invention to obtain a primarycoating having an equilibrium modulus of about 1.5 MPa or less andhaving a low actual stress level.

SUMMARY OF THE INVENTION

The present inventors realized that the integrity of a soft primarycoating in a coated optical fiber during use is (a.o.) dependent on itsresistance to cavitation.

Therefore, the present invention relates to a coated optical fiberhaving a primary coating which sufficiently adheres to the optical fiberto reduce to a minimum the occurrence of delaminations (or debonding) atthe primary coating-glass interface and wherein the secondary coatingsufficiently adheres to the primary coating to reduce to a minimum theoccurrence of delaminations at the primary coating-secondary coatinginterface, wherein said primary coating has a cavitation strength thatis sufficient to reduce to a minimum the occurrence of cavitationswithin the coating itself.

Therefore, the present invention provides a coated optical fiber havinga primary coating having a storage modulus at 23° C. (E′₂₃), having anequilibrium modulus of about 1.5 MPa or less and having a cavitationstrength of at least about 1.40 times its storage modulus at 23° C.(E′₂₃), but said cavitation strength having a value of at least 1.0 MPa,while having sufficient adhesion to glass.

A suitable definition of the phenomenon of cavitation strength accordingto the present invention is the stress at which the tenth cavitationbecomes visible when measured in a tensile testing machine at a pullingspeed of 20 μm/min for a 100 μm thin layer (or 20% per min) whenobserved at a magnification of about 20×.

The present invention furthermore provides a coated optical fibercomprising said primary coating and a secondary coating having a Tg ofabout 40° C. or more and a modulus (1 Hz; storage modulus E′ at 23° C.)of about 400 MPa or more.

The present invention furthermore provides a primary coatingcomposition, when cured, having above defined cavitation strength,furthermore the invention provides a primary coating having sufficientstrain hardening to substantially increase the resistance to cavitationof the primary coating in comparison to a coating exhibiting “idealGaussian rubber” characteristics and/or said primary coating having asufficient strain energy release rate (Go).

Further, the present invention provides a primary coating having asufficient low expansion coefficient while having a low modulus and animproved combination of expansion coefficient for a primary-secondarycoating system.

The present invention further provides an apparatus for measuring thecavitation strength of a coating and a method for measuring saidcavitation strength of a coating for use as a primary coating on anoptical glass fiber.

SHORT DESCRIPTION OF PHOTOGRAPHS AND FIGURES

Photograph 1 shows the set up for a cavitation strength measurement.

Photograph 2 shows the top fixture of the cavitation strengthmeasurement set up.

Photograph 3 shows sample of two primary coatings with cavities.

Photograph 4 shows the micrometer set-up used for the sample preparationfor the cavitation strength measurement.

FIG. 1 schematically shows an apparatus used for determining thecavitation strength of a sample.

FIG. 2 shows the sample geometry in the cavitation set-up.

FIG. 3 shows the cavitation strength at the tenth cavitation as afunction of E′23.

FIG. 4 shows the number of cavitations at increasing stresses on aprimary coating sample with precure (0.96 J/cm²+3 precure flashes) and asample without precure (0.93 J/cm²) (speed 20%/min).

FIG. 5 shows relative Mooney plots of several primary coatings.

DETAILED DESCRIPTION OF THE INVENTION

The primary coating of the present invention has an equilibrium modulusof about 1.5 MPa or less. The equilibrium modulus according to thepresent invention is measured by DMTA in tension according to ASTMD5026-95a, wherein the modulus is determined as described in theexperimental section. Use of such a low modulus primary coating resultsin an increased resistance against attenuation of the light transportedthrough the glass fiber. This resistance against attenuation is inparticular relevant in so called “non zero dispersion shifted singlemode optical fibers”, and in multimode fibers as these fibers aresensitive to attenuation due to so-called microbending.

Preferably, the equilibrium modulus is about 1.3 MPa or less, morepreferred about 1.0 MPa or less, even more preferred about 0.9 MPa orless, and most preferred, about 0.8 MPa or less. In general, the moduluswill be about 0.05 MPa or higher, preferably about 0.1 MPa or higher,more preferably about 0.2 MPa or higher, and most preferred, about 0.3MPa or higher.

In spite of the low modulus, the resistance to cavitation (furthercalled cavitation strength) should be sufficiently high. The presentinvention now provides primary coating compositions, which when curedresult in primary coatings fulfilling the above requirements.

The present invention also provides a method and an apparatus formeasuring the cavitation strength, which is the stress at which adefined number of cavitations becomes visible at about 20×magnification. For the purpose of the present invention the stress ismeasured at which a second, fourth, or tenth cavitation becomes visibleat about 20× magnification at a pulling speed of 20 μm/min in a 100 μmthick sample (or 20% min⁻¹).

This method and apparatus can then be used to design the primarycoatings of the present invention.

The apparatus for measuring the cavitation strength of a coatingaccording to the present invention comprises an assembly comprising:

-   -   (i) a first member having a first surface;    -   (ii) a second member having a second surface opposing said first        surface; at least one of said first and said second memberbeing        transparent to ultraviolet light; said first surface being        moveable in a direction normal towards said second surface; said        first surface defining with said second surface a cavity for        receiving a sample;    -   (iii) a sub-assembly in contact with said first member or said        second member; said sub-assembly comprising at least one element        capable of adjusting the position of said first surface or said        second surface in such a manner that said first surface or said        second surface is perpendicular to the direction of said normal        movement (see FIG. 1).

FIG. 1 schematically shows an apparatus that may be used for determiningthe cavitation strength of a sample (30), in particular an ultravioletcured film of ultraviolet curable material. The set up includes atensile testing apparatus comprising an assembly for holding a samplefor testing.

The tensile testing apparatus comprises a load cell (50) for measuringthe force that is required to move a moving plate (80) in a normaldirection apart from a stationary plate (70). Load cell (50) is attachedto the stationary plate (70). The movement of the plate (80) may beguided by bars or a set of bars (100). The tensile testing apparatus mayfurther comprise a displacement transducer (90), which can regulate thespeed with which the plate (80) is displaced from the stationary plate(70).

The assembly comprises a first member (10) having a first surface and asecond member (20) with a second surface facing said first surface.Thus, said load cell (50) can register the force that is required tomove said first surface in a direction normal towards said secondsurface and said displacement transducer can regulate the speed at whichsaid first surface is normally moved from said second surface. Together,the first and second surface define an area for holding a sample (30).Preferably at least one of the first member and second member is made ofa material that is transparent to ultraviolet (UV) light. Materials thatare transparent to UV light are well-known in the art and include, forinstance, quartz glass. Preferably at least the second member (20) ismade of UV transparent material. Preferably, the assembly is capable ofreceiving a UV curable composition which may be cured in situ.

Generally, both the first and the second member are transparent to thenaked eye. The second member is adapted to be attached to load cell(50).

The assembly further comprises a sub-assembly for positioning the firstmember (10) in a position perpendicular to the direction of the farceapplied to the sample (30). The sub-assembly has at least one element(40) that can adjust the position of the first member (10) relative tothe direction in which the moving plate (80) is displaceable. Such anelement may be, for instance, an adjustment screw. Preferably, thesub-assembly comprises at least two adjustment screws, more preferablyat least three adjustment screws, and most preferred, at least threemicrometer screws on the moving plate and three hardened steel ballsfitted to the adjustable plate. The sub-assembly further comprises aring plate (110) attached to the moving plate (80). The plate ispreferably constructed such that it is sufficiently rigid to minimize oreliminate any effect on the measuring of the sample during testing. Forexample, the plate (80) may be constructed from rigid steel. A boreextends through the plate (110), and also through the moving plate (80),to allow the sample (30) to contact both said first surface of the firstmember (10) and said second surface of the second member (20). Firstmember (10) is attached to or rests on the ring plate (110). An exampleof how the sub-assembly can adjust the position of the first member (10)relative to the movement of the plate (80) is given below:

In operation, the adjustment elements (40) in FIG. 1 could be, forexample, adjustment screws, such as micrometer screws. Adjusting one ofthe screws will cause the ring plate (110) to change its angle (or tilt)with respect to the direction of movement (or force imposed on thesample during testing). Since the first member (10) is attached to orrests on the ring plate (110), the first member (10) will also changeits angle (or tilt) relative to moving plate (80). Accordingly, theposition of the first member (10) relative to the moving direction ofthe moving plate (80) is adjusted. One of the benefits of thesub-assembly is that it be used to ensure that the position of saidfirst surface of the first member (10) is perpendicular to the movingdirection of the moving plate (80).

Preferably, said sub-assembly is capable of adjusting the position insuch a manner that both the first member and the second member areperpendicular to the direction of the normal movement or movingdirection of the moving plate (80) or, in other words, are parallel toeach other (further called parallelity adjustment).

The set up in FIG. 1 further comprises a viewer (60) for opticallyobserving and/or recording a contacting surface of the sample (30) in adirection parallel to the moving direction of the moving plate (80).Such viewer (or viewing means) (60) may be any device suitable forobserving the surface contacting either the first or second surfaceand/or the sample in between. Preferably, the viewer includes amagnifier, such as, for instance, a microscope, a video camera, and/or amicroscope in conjunction with a video camera.

The present invention further relates to a tensile testing apparatuscomprising the assembly as described above. Said tensile testingapparatus comprising said assembly has a compliance of less than about0.5 μm/N, preferably, of about 0.4 μm/N or less, more preferred, about0.3 μm/N or less, and most preferred about 0.2 μm/N or less.

The details of the apparatus for measuring the cavitation strength arefurther visualized in Photograph 1. In particular, the apparatus is usedfor measuring the cavitation strength of a primary optical glass fibercoating and comprises, a tensile testing machine having a fixed plate towhich a load cell with a lower end sample part (second member (20)) canbe fixed, optionally further comprising a displacement transducer, andcomprising a moving plate and a top fixture; either the top (firstmember (10)) sample part or lower sample part (second member (20)) beingprovided with means to adjust the parallelity of the sample to beperpendicular to the direction of the normal movement (see Photograph2), the apparatus being further provided with a microscope andpreferably also a recorder fitted on said top (moving) plate, thecompliance of the total set up of the apparatus being less than about0.5 μm/N (preferred ranges see above) and wherein the thickness of saidtop and lower sample part are about 2 mm or more, preferably, about 3 mmor more, more preferably, about 4 mm or more.

The method for measuring the cavitation strength according to thepresent invention comprises the steps of:

-   (i) making a sample by treating two plates (each having a thickness    of at least 5 mm), preferably at least one quartz plate, by applying    a liquid coating in between the two plates in a thickness of between    10 and 300 μm and over a certain area and by curing said coating    with a UV-dose, the treatment of the two plates being such that the    adhesion between the plates and the cured coating is sufficient to    obtain cavitation before debonding sets in,-   (ii) placing the sample in a tensile testing apparatus, which is    provided with a microscope, in such a way that a substantially    parallel alignment and an acceptable compliance of the total tensile    testing apparatus is obtained,-   (iii) running a deformation test on said sample while measuring the    force at which a defined number of cavities starts to be visible    through the microscope at a certain magnification, and-   (iv) calculating the stress by dividing said force by the area of    the coating applied and reporting said stress in relation to said    cavities.

Preferably, the coating is cured with such UV-dose that the coatingattains at least 85% of its equilibrium modulus (preferably, at least90%, more preferred, at least 95%). It is preferred to cure the coatingwith a UV-dose of about 1 J/cm².

“Debonding” means interfacial failure at the interface between coatingand plate. The treatment of the two plates preferably consists oftreating the surface with a silane solution containing a silane couplingagent, more preferably by first finely polishing the surface usingcarborundum powder, most preferred, the treatment as indicated in thedescription of the test methods under paragraph A.2.

According to a preferred embodiment of the present invention the methodfor measuring the cavitation strength comprises the steps of:

-   (i) making a sample by-   (a) cleaning two plates, preferably glass or quartz plates, more    preferred, at least one of which is a quartz plate, each having a    thickness of at least 5 mm,-   (b) preparing the surfaces of said plates, preferably by roughening    them,-   (c) treating the surfaces thereof with a silane coupling agent,-   (d) providing a coating material between the two plates of at least    0.1 square cm in area, preferably 0.2-1 square cm, in a thickness of    between 10-300 μm, preferably of about 100 μm, and-   (e) curing said coating with UV light in an amount of about 1 J/cm²;-   (ii) placing the sample in a tensile testing apparatus which is    provided with a microscope, and preferably a video recorder,-   (iii) running a deformation test at a speed of 0.05-1.00 min⁻¹,    preferably 0.1-0.5 min⁻¹, and most preferred 0.15-0.25 min⁻¹, while    measuring the force at which a defined number of cavities starts to    be visible through the microscope at 20× magnification, and-   (iv) calculating the stress by dividing said force by said area and    reporting said stress in relation to said cavities.

Photograph 3 shows the appearance of cavities in two samples of primarycoatings A and B in a cavitation measurement as a function of the forceapplied. The cavities can have different forms depending on the type ofprimary coating. Coating A shows bubble-like cavities whereas coating Bshows stripe-like cavities.

The measurement preferably is performed by videorecording the sampleduring the measurement. The measurement can be performed with a 100 μmthin layer, for which the pulling speed of 20 μm/min can be used toobtain a deformation rate of 0.20 min⁻¹. The deformation rate can bedefined by the pulling speed divided by the layer thickness.

Preferably, in the test for cavitation strength, the stress at which,the second, fourth or tenth cavitation becomes visible is taken as thecavitation strength of a coating. In the present invention the tenthcavitation is used as the measuring point.

The cavitation strength at which the tenth cavity appears (σ¹⁰ _(cav))preferably is 1.0 MPa or higher as measured at a deformation rate of0.20 min⁻¹, of a primary coating sample which has been preparedaccording to the method described in detail in the experimental sectionand the cavitation strength preferably is at least 1.4 times the storagemodulus at 23° C. (E′₂₃) of said primary coating (see FIG. 3).

Therefore, the present invention relates to a primary coatingcomposition when cured having an equilibrium modulus of about 1.5 MPa orless and a cavitation strength at which a tenth cavitation appears (σ¹⁰_(cav)) of at least about 1.0 MPa as measured at a deformation rate of0.20% min⁻¹, said cavitation strength being at least about 1.4 timessaid storage modulus at 23° C. (E′₂₃). Preferably, the cavitationstrength is at least about 1.5 times the storage modulus, morepreferably at least about 1.6 times the storage modulus.

This can be achieved in several ways, as explained below in more detail.One way is by introducing strain hardening into the material, such as byintroducing bimodality (or multimodality) into the system, or byintroducing crystallization under strain. Another way of increasing theresistance to cavitation is by using a two step curing processcomprising a first low dose pre-cure step.

The stress which is excerted (in actual use) on the primary coatingfurther depends on the secondary coating. Also, the time over which thestress is exerted has an influence because relaxation can reducestresses. The latter is shown e.g. by Reddy et al., in the 1993 Proc. ofthe 42^(nd) WCS p. 386-392. A higher modulus and higher Tg secondarycoating will cause greater stress on the primary. Hence, it is preferredthat the cavitation strength of the primary coating at which the tenthcavitation σ¹⁰ _(cav) appears is about 1.1 MPa or more, and morepreferred about 1.2 MPa or more, and most preferred about 1.3 MPa ormore.

The present invention further relates to a coated optical fibercomprising a glass optical fiber, a primary coating applied thereon, asecondary coating and optionally an ink composition subsequently appliedthereon, wherein the primary coating is as defined above.

In order to improve the resistance of the primary coating againstcavitations to occur, two characteristics appear to be important. Acoating should preferably have a strain hardening behavior, and thecoating preferably should have a certain strain energy release rate(Go).

Strain hardening can be defined by the behavior of a coating in atensile test resulting in a stress-strain curve that deviates from an“ideal rubber”-profile, as explained further below. Strain hardening canbe measured by a stress-strain curve, and is preferably defined by acurve in a relative Mooney plot (see FIG. 5), as described below.

A relative Mooney plot can be obtained as follows:

The primary measurement is the force-displacement curve, measuredaccording to ISO 37 with a speed of 5 mm/min, preferably 50 mm/min, andmore preferred 500 mm/min. At higher speed, it is more certain that theeffect of the material can be measured, in particular, when the strainhardening behaviour sets in only at higher strain. From this measurementthe engineering stress can be calculated according to formula (1);

$\begin{matrix}{\sigma_{E} = \frac{F}{A}} & (1)\end{matrix}$where F is the force and A is the initial cross-section of the sample.The strain λ is calculated by formula (2):

$\begin{matrix}{\lambda = \frac{l}{l_{0}}} & (2)\end{matrix}$where l₀ is the initial length and l the actual length of the prismaticregion of the sample under test.

The Mooney stress, σ_(M), can now be calculated from this engineeringstress using (“Elastomers and Rubber Elasticity”, J. E. Mark and J. Lal,1982, ACS Symposium Series 193, American Chemical Society WashingtonD.C.):

$\begin{matrix}{\sigma_{M} = \frac{\sigma_{E}}{\lambda - \frac{1}{\lambda^{2}}}} & (3)\end{matrix}$

A Mooney plot can now be constructed by plotting σ_(M) versus 1/λ. Astrain hardening material shows an increase in the relatively Mooneystress at lower values of 1/λ. On the contrary, an ideal rubber materialdoes not show this increase in the relatively Mooney stress at lowervalues of 1/λ. Since the engineering stress for an ideal rubber materialis given by the formula

$\begin{matrix}{\sigma_{E} = {E\left( {\lambda - \frac{1}{\lambda^{2}}} \right)}} & (4)\end{matrix}$wherein E is the equilibrium modulus, the Mooney stress σ_(M) of anideal rubber material is and remains equal to E upon increasing strainλ.A strain hardening material behaves as a finite extendable spring. Itshows a linear elastic behavior under an initial, small strain, butdevelops a limit in its stretching capacity upon further increase of thestrain. From that point, a much higher stress is then required tofurther stretch the material, and thus, for a cavity to develop andgrow. In a relative Mooney plot, this is visible by a rather steepincrease in the relative Mooney stress at increasing strain λ.

To be able to compare different materials, the relative Mooney stress isfurther used. The relative Mooney stress, σ_(rM), is now defined asfollows. First, determine the minimum of σ_(M) for 1/λ≦0.8, which isfurther denoted as σ_(M,min). Then the relative Mooney stress is givenby:

$\begin{matrix}{\sigma_{rM} = \frac{\sigma_{M\;}}{\sigma_{M,{m\; i\; n}}}} & (5)\end{matrix}$

The relative Mooney plot can now be constructed by plotting σ_(rm)versus 1/λ.

The curve f(λ) in the relative Mooney plot (see FIG. 5) will be used todefine the primary coatings of the present invention:

$\begin{matrix}{{f(\lambda)} = {a\;\frac{{L^{- 1}\left( \frac{\lambda}{\sqrt{b}} \right)} - {\lambda^{- \frac{3}{2}}{L^{- 1}\left( \frac{1}{\sqrt{\lambda}\sqrt{b}} \right)}}}{\lambda - \frac{1}{\lambda^{2}}}}} & (6)\end{matrix}$where L⁻¹(x) is the inverse of the Langevin function L(x) (“The physicsof rubber elasticity”, L. R. G. Treloar, second edition, 1967, Oxford atClarendon press), which is defined as:

$\begin{matrix}{{L(x)} = {{\coth(x)} - \frac{1}{x}}} & (7)\end{matrix}$The constants a and b are respectively 0.94 and 11.20.

The primary coatings showing strain hardening according to the presentinvention show a curve in the relative Mooney plot which increases onlowering 1/λ and of which at least one part has a value higher than thevalue calculated by using the function f(λ) for 1/λ of about 0.60 orless.

Preferably, at least one part of said curve has a value higher than thevalue calculated by using f(λ) wherein a=0.86 and b=9.85 for 1/λ ofabout 0.60 or less.

More preferably, at least one part of said curve has a value higher thanthe value calculated by using f(λ) wherein a=0.78 and b=8.50 for 1/λ ofabout 0.60 or less.

Most preferred, at least one part of said curve has a value higher thanthe value calculated by using f(λ) wherein a=0.70 and b=7.15 for 1/λ ofabout 0.60 or less.

The above values for f(λ) preferably apply for 1/λ of about 0.55 orless, more preferably, for 1/λ of about 0.50 or less.

According to a preferred embodiment of the present invention, the strainhardening behavior of a primary coating is more effective in preventingcavities if the strain hardening occurs at lower elongation (or higher1/λ).

In De Vries et al., Journal of Polymer Science: Polymer Symposium 58,109-156 (1977) it is described to obtain a stress-strain curve from abiaxial stretching test wherein the applied strain is biaxial incontrast to the uniaxial experiment performed in the above.

For the sake of simplicity, and in order to obtain less noisy profiles,the present inventors have used an uniaxial stretching test wherein thecoating test specimen is pulled in uniaxial direction according to ISO37 under the conditions as described in the experimental section.

One of the other preferred characteristics of the primary coatings ofthe present invention is to have a certain strain energy release rateGo. The strain energy release rate or tear strength (Go), is the energyrequired per 1 m² of crack surface in a test specimen of a cured primarycoating initially containing a small crack equal to the slit length b asdefined in ISO 816.

The strain energy release rate Go depends on the strain rate in asimilar manner as the cavitation strength. The strain energy releaserate is preferably at least about 20 J/m², as measured at a rate ofabout 1.10⁻⁵ s⁻¹ or less. A higher tear strength aids in precluding theoccurring of cavities, in particular if the coating already shows somestrain hardening behavior.

Therefore, according to a preferred embodiment of the present invention,the primary coating having an equilibrium modulus of about 1.5 MPa orless, when measured in an uniaxial tensile test and represented in arelative Mooney plot, shows a curve which increases on lowering 1/λ, andof which at least one part has a value higher than the value calculatedby using the function f(λ) for 1/λ, of about 0.60 or less wherein a=1.17and b=15.0 and said primary coating has a strain energy release rate Go,as measured at a rate of about 1.10⁻⁵ s⁻¹ or less, of higher than55.0−24.0×E_(equilibrium).

A tear strength Go of over about 150 J/m² generally does not furtherincrease the cavitation strength of a coating. However, the tearstrength preferably is about 30 J/m² or more, more preferably about 35J/m² or more, particularly preferred about 40 J/m² or more, and mostpreferred about 45 J/m² or more. These Go values preferably apply toprimary coatings showing strain hardening by a curve in the relativeMooney plot which increases on lowering 1/λ and of which at least onepart of said curve has a value higher than the value calculated by usingf(λ) wherein a=1.02 and b=12.55 for 1/λ of about 0.60 or less, morepreferably wherein a=0.94 and b=11.20 for 1/λ of about 0.60 or less.

Preferably, the primary coating has an equilibrium modulus of about 1.2MPa or less, more preferably, about 1.0 MPa or less, even morepreferably about 0.9 MPa or less, and most preferred about 0.8 MPa orless.

With the primary coatings of the present invention it is possible tomake coatings which have a very low modulus and yet have a high level ofintegrity with respect to cavitation strength.

Furthermore, this invention allows for the design of coating systems inwhich the secondary coating has a high Tg and/or high storage modulus at23° C. and the primary has a (very) low equilibrium modulus (preferably,about 1.2 MPa or less, more preferably, about 1.0 MPa or less, even morepreferably about 0.9 MPa or less, most preferred about 0.8 MPa or less).The Tg of primary coatings generally is less than about 0° C.,preferably, less than about −5° C., more preferred, less than about −10°C., and most preferred, less than about −20° C. (as measured by thefirst peak tan-δ at 1 Hz in a DMA curve when starting from the hightemperature side). In general, the Tg of primary coatings is at leastabout −80° C., preferably at least about −60° C. High modulus secondarycoatings are desirable for certain cable designs. Generally, the Tg ofthe secondary (as measured by the peak tan-δ in DMTA) is about 40° C. orhigher. Preferably, the Tg is about 50° C. or higher, and morepreferable about 60° C. or higher. Generally, the Tg will be about 100°C. or lower. The storage modulus E′ at 23° C. preferably is about 200MPa or higher, more preferably between 400-3000 MPa.

The primary coating generally will be a radiation curable coating basedon (meth)acrylate functional oligomers and radiation-curable monomerswith photoinitiator(s) and additives. Examples of additives include astabiliser and a silane coupling agent. The adhesion to the glass asmeasured according to adhesion test described in WO 99/15473, which isincorporated herein by reference, generally is at least about 5 g inforce at 50% RH and at 95% RH (Relative Humidity). Preferably, theadhesion is at least about 10 g in force, more preferably at least about20 g in force, particularly preferred at least about 50 g in force andmost preferred at least about 80 g in force, both at 50% RH and 95% RH.The adhesion may be as high as 250 g in force.

The radiation curable coatings of the present invention generallycomprise

(A) 20-98% by wt. of at least one oligomer having a molecular weight ofabout 1000 or higher, preferably, 20-80% by wt., more preferably, 30-70%by wt.,

(B) 0-80% by wt. of one or more reactive diluents, preferably, 5-70% bywt., more preferably, 10-60% by wt., most preferably, 15-60% by wt.,

(C) 0.1-20% by wt. of one or more photoinitiators for initiation of aradical polymerisation reaction, preferably, 0.5-15% by wt., morepreferably, 1-10% by wt., most preferably, 2-8% by wt.,

(D) 0-5% by wt. of additives,

wherein the total amount adds up to 100 wt. %.

Preferably, the oligomer (A) is a urethane (meth)acrylate oligomer,comprising a (meth)acrylate group, urethane groups and a backbone.(Meth)acrylate includes acrylate as well as methacrylate functionality.The backbone is derived from a polyol which has been reacted with adiisocyanate and hydroxy alkyl acrylate. However, urethane-freeethylenically unsaturated oligomers may also be used.

Examples of suitable polyols are polyether polyols, polyester polyols,polycarbonate polyols, polycaprolactone polyols, acrylic polyols, andthe like. These polyols may be used either individually or incombinations of two or more. There are no specific limitations to themanner of polymerization of the structural units in these polyols. Anyof random polymerization, block polymerization, or graft polymerizationis acceptable. Examples of suitable polyols, polyisocyanates andhydroxyl group-containing (meth)acrylates are disclosed in WO 00/18696,which is incorporated herein by reference.

The reduced number average molecular weight derived from the hydroxylnumber of these polyols is usually from about 50 to about 25,000,preferably from about 500 to about 15,000, more preferably from about1,000 to about 8,000, and most preferred, from about 1,500 to 6,000.

The ratio of polyol, di- or polyisocyanate (as disclosed in WO00/18696), and hydroxyl group-containing (meth)acrylate used forpreparing the urethane (meth)acrylate is determined so that, about 1.1to about 3 equivalents of an isocyanate group included in thepolyisocyanate and about 0.1 to about 1.5 equivalents of a hydroxylgroup included in the hydroxyl group-containing (meth)acrylate are usedfor one equivalent of the hydroxyl group included in the polyol.

In the reaction of these three components, an urethanization catalystsuch as copper naphthenate, cobalt naphthenate, zinc naphthenate,di-n-butyl tin dilaurate, triethylamine, and triethylenediamine,2-methyltriethyleneamine, is usually used in an amount from about 0.01to about 1 wt % of the total amount of the reactant. The reaction iscarried out at a temperature from about 10 to about 90° C., andpreferably from about 30 to about 80° C.

The number average molecular weight of the urethane (meth)acrylate usedin the composition of the present invention is preferably in the rangefrom about 1,200 to about 20,000, and more preferably from about 2,200to about 10,000. If the number average molecular weight of the urethane(meth)acrylate is less than about 1000, the resin composition tends tovitrify at room temperature; on the other hand, if the number averagemolecular weight is larger than about 20,000, the viscosity of thecomposition becomes high, making handling of the composition difficult.

The urethane (meth)acrylate is preferably present in an amount fromabout 20 to about 80 wt %, of the total amount of the resin composition.When the composition is used as a coating material for optical fibers,the range from about 20 to about 80 wt % is particularly preferable toensure excellent coatability, as well as superior flexibility andlong-term reliability of the cured coating.

Preferred oligomers are polyether based acrylate oligomers,polycarbonate acrylate oligomers, polyester acrylate oligomers, alkydacrylate oligomers and acrylated acrylic oligomers. More preferred arethe urethane-containing oligomers thereof. Even more preferred arepolyether urethane acrylate oligomers and urethane acrylate oligomersusing blends of the above polyols, and particularly preferred arealiphatic polyether urethane acrylate oligomers. The term “aliphatic”refers to a wholly aliphatic polyisocyanate used. However, alsourethane-free acrylate oligomers, such as urethane-free acrylatedacrylic oligomers, urethane-free polyester acrylate oligomers andurethane-free alkyd acrylate oligomers are preferred.

Suitable reactive diluents (B) are polymerizable monofunctional vinylmonomers and polymerizable polyfunctional vinyl monomers.

These reactive diluents are disclosed in WO 97/42130, which isincorporated herein by reference.

These polymerizable vinyl monomers are preferably used in an amount fromabout 10 to about 70 wt %, and more preferred from about 15 to about 60wt %, of the total amount of the resin composition.

Preferred reactive diluents are alkoxylated alkyl substituted phenolacrylate, such as ethoxylated nonyl phenol acrylate, propoxylated nonylphenol acrylate, vinyl monomers such as vinyl caprolactam, isodecylacrylate, and alkoxylated bisphenol A diacrylate such as ethoxylatedbisphenol A diacrylate.

Preferably, the photoinitiators (C) are free radical photoinitiators.

Free-radical photoinitiators are generally divided into two classesaccording to the process by which the initiating radicals are formed.Compounds that undergo unimolecular bond cleavage upon irradiation aretermed Type I or homolytic photoinitiators.

If the excited state photoinitiator interacts with a second molecule (acoinitiator) to generate radicals in a bimolecular reaction, theinitiating system is termed a Type II photoinitiator. In general, thetwo main reaction pathways for Type II photoinitiators are hydrogenabstraction by the excited initiator or photoinduced electron transfer,followed by fragmentation.

Examples of suitable free-radical photoinitiators are disclosed in WO00/18696 and are incorporated herein by reference.

Preferably, the total amount of photoinitiators present is between about0.10 wt. % and about 20.0 wt. % relative to the total amount of thecoating composition. More preferably, the total amount is at least about0.5 wt. %, particularly preferred, at least about 1.0 wt. %, and mostpreferred, at least about 2.0 wt. %. Moreover, the total amount ispreferably less than about 15.0 wt. %, more preferably, less than about10.0 wt. %; and particularly preferred, less than about 6.0 wt. %

In one preferred embodiment of the present invention at least one of thephotoinitiators contains a phosphorous, sulfur or nitrogen atom. It iseven more preferred that the photoinitiator package comprises at least acombination of a photoinitiator containing a phosphorous atom and aphotoinitiator containing a sulfur atom.

In another preferred embodiment of the invention, at least one of thecompounds (C) is an oligomeric or polymeric photoinitiator.

As an additive (D), an amine compound can be added to the liquid curableresin composition of the present invention to prevent generation ofhydrogen gas, which causes transmission loss in the optical fibers. Asexamples of the amine which can be used here, diallylamine,diisopropylamine, diethylamine, diethylhexylamine, and the like can begiven.

In addition to the above-described components, various additives such asantioxidants, UV absorbers, light stabilizers, silane coupling agents,coating surface improvers, heat polymerization inhibitors, levelingagents, surfactants, colorants, preservatives, plasticizers, lubricants,solvents, fillers, aging preventives, and wettability improvers can beused in the liquid curable resin composition of the present invention,as required.

The description can also apply to colored primary coating compositions.The colorant can be a pigment or dye, preferably, a dye.

Radiation curable primary coating compositions are described in forexample: EP-A-0565798, EP-A2-0566801, EP-A-0895606, EP-A-0835606 andEP-A-0894277.

In particular low modulus coatings are described in WO99/08975, inWO99/52958, in WO91/03499, and in EP-B1-566801.

The content of these references is incorporated herein, as thesereferences provide the man skilled in the art sufficient information tomake low modulus coatings.

The zero shear viscosity at 23° C. of the liquid curable resincomposition of the present invention is usually in the range from about0.2 to about 200 Pa·s, and preferably from about 2 to about 15 Pa·s.

The elongation-at-break of the primary coatings of the present inventionis typically greater than about 50%, preferably greater than about 60%,more preferably the elongation-at-break is at least about 100%, morepreferably at least about 150% but typically not higher than about 400%.This elongation-at-break can be measured at a speed of 5 mm/min, 50mm/min or 500 mm/min respectively, preferably at 50 mm/min.

In order to make the effect of strain hardening visible in the presentuniaxial test, the elongation-at-break of the primary coatings shouldpreferably be at least about 100%.

According to one embodiment of the present invention one way ofimproving the cavitation strength is by introducing bimodal (ormultimodal) distribution of the molecular weight of the multifunctionalcross-linking components (further called bi- or multimodality), in otherwords, by a primary coating composition comprising at least onecross-linking component introducing bimodality into the system.Bimodality means that the system network contains chains of at least twodifferent lengths between the junctions of the network.

In contrast to the normal practice in radiation curable oligomersynthesis wherein the low Mw-fractions are restricted to a minimum oravoided, it is preferred according to the present invention to modifythe Mw-distribution by introducing a sufficient amount of a low Mwoligomer or multifunctional monomer to obtain the desired cavitationstrength and/or strain hardening.

This can be achieved (a.o.) by using at least two oligomers, preferably,oligomer diacrylates, with a different average molecular weight,preferably the average molecular weight of the one oligomer being onaverage 2 times higher than the molecular weight of the other oligomer,more preferably being on average at least 5 times higher, mostpreferred, at least 10 times higher; another possibility is by using atri- or tetrafunctional oligomer, still another option is by using threeoligomers with a different average molecular weight, further calledtrimodality. In the latter case, it is preferred that the averagemolecular weight of the one oligomer is on average 1.5 times higher thanthe molecular weight of the second oligomer and the average molecularweight of the second oligomer is on average 1.5 times higher than theaverage molecular weight of the third oligomer, more preferably theaverage molecular weight between the oligomers doubles, most preferably,the average molecular weight of the three oligomers differs at least 5times. An alternative option—which is preferred—is by using amultifunctional (for example difunctional or multifunctional) reactivediluent in an amount sufficient to achieve the desired cavitationstrength characteristics. A further option is to use a combination of atleast two di- or multifunctional acrylates of low Mw (reactivediluents).

Mori et al. describe (in RadTech proceedings 1998, USA) the use of up to4% of difunctional acrylate diluents in a primary coating. Thesecoatings however, have a too high equilibrium modulus.

EP-A-0311186 and EP-A-0167199 describe the use of 6% or 9.5% of adifunctional acrylate in a low modulus primary. However, these coatingsshow a decrease in equilibrium modulus after aging at 95° C. for 30 daysof more than 60%. Moreover, these coatings show strong yellowing uponaging under fluorescent light for 30 days. None of the referencessuggest anything on resistance to cavitation.

The primary coatings of the present invention show a decrease inequilibrium modulus after aging for 30 days at 95° C. of less than 50%,preferably, less than 45%, more preferably, less than 40%. Preferably,their E′₁₀₀₀ (temperature at which their storage modulus equals 1000MPa) decreases by less than about 10° C. under the above agingconditions, more preferably, less than about 7° C.; their E′100(temperature at which their storage modulus equals 100 MPa) decreases byless than about 20° C. under the above aging conditions, morepreferably, less than about 15° C.

Preferably, the primary coatings of the present invention show, uponaging for 30 days in fluorescent light (4 mW/cm²), a non-yellowing valueDE of about 20 or less, more preferably, about 15 or less. The 30 daysaging test was performed using a daylight L 35 W/11 Lumilux lampavailable from Osram, at such a distance that the energy at the surfaceof the coating is 4 mW/cm² as measured using a ML 1400 radiometeravailable from Miltec comprising an IL 1740B Photoresist. The colorchange delta E value of the cured films is measured by conventionalmethods as disclosed in the publication entitled “A Measurement of theContribution of UV Cured Coatings and Ink Binders Towards Color Changeof UV Cured Inks” by D. M. Szum in Radtech Europe '93 ConferenceProceedings (papers presented at the Radtech Europe Conference held May2-6, 1993), the complete disclosure of which is hereby incorporated byreference. This publication discloses measurements which were performedon three layer samples, whereas the samples of the present inventionwere single layers. The measurement involves a mathematicalmanipulation, FMC-2.

According to a further preferred embodiment of the present invention theprimary coatings (75 μm films cured in nitrogen at 1 J/cm² using one Dlamp; UV-dose determined with a “Light Bug” manufactured byInternational Light, Inc.; wavelengths measured 257-390 nm) show ahydrogen generation (24 hours at 80° C. in argon) of about 0.3 μl/g orless, more preferred, about 0.25 μl/g or less, even more preferred,about 0.20 μl/g or less.

Another preferred way of increasing the resistance against cavitation isto lower the amounts of non-load bearing material, more in particular,of monofunctional (low Mw) acrylates, generally having a Mw below about1000, more preferably below about 700, even more preferably below about600, particularly preferred below about 500, most preferred below about400. The amount of monofunctional acrylate preferably is about 10 wt. %or less, more preferably about 8 wt. % or less, even more preferredabout 5 wt. % or less, particularly preferred about 4 wt. % or less, andmost preferred about 3 wt. % or less. The monofunctional acrylate ispreferably present in an amount of at least about 0.5 wt. %, morepreferably at least about 1 wt. %, even more preferred at least about1.5 wt. %. Any modulus increase can be compensated by increasing themolar mass of the oligomer diacrylate. Preferably, this measure iscarried out in addition to introducing a certain amount of strainhardening (such as for example by introducing bimodality) into thecoating system.

Both strain hardening and tear strength are increased by introducing abimodal coating composition. It is preferred to use a sufficient amountof a difunctional component with a molecular weight of about 1000 orless to obtain the required strain hardening or tear strength. Theamount of low molecular weight multifunctional diluent [preferablydifunctional diluent, trifunctional diluent, long chain trifunctionaldiluent or a combination thereof] preferably is about 1.6 wt % orhigher, more preferably about 1.8 wt % or higher, most preferred about2.5 wt % or higher. Generally, the amount will be less than about 15 wt%, preferably less than about 9 wt % if the molecular weight of thedifunctional diluent is less than about 500. Alkoxylated dioldiacrylates are preferred in the coatings of the present invention.

Suitable examples of dioldiacrylates include hexanediol diacrylate,ethoxylated bisphenol-A diacrylate, tripropylene glycoldiacrylate andthe like. The at least one oligomer preferably has a molecular weight ofabout 4000 or more, more in particular of about 5000 or more. Generally,in view of viscosity requirements, the molecular weight is about 20,000or less, preferably about 15,000 or less, more preferably about 10,000or less. Any oligomer can be used, but wholly aliphatic polyetherurethane oligomers are preferred. Also, polyether/polyester andpolyether/polycarbonate combined urethane acrylate oligomers arepreferred.

Another way of introducing the desired strain hardening into the coatingof the present invention is by introducing crystallization under strain.Upon stretching the network, the network chains crystallize and thusstiffen the material in situ.

The resistance to cavitation can also be improved by a two step curingprocess, in which the coating is partly cured with a very low first dose(5-50 mJ/cm², hereinafter called pre-cure), and thereafter cured with adose of at least about 50 mJ/cm². The time period between the first andsecond dose preferably is 2-120 sec. On a draw tower, the time periodbetween the first and subsequent doses preferably is much shorter,preferably between 1.10⁻³ and 5 sec. Therefore, the time period betweenthe first and subsequent doses preferably is between about 1.10⁻³ andabout 120 sec. It is preferred to pre-cure the coating with one or moreshort flashes of a UV-source resulting in a total dose of about 0.01J/cm² or less (see FIG. 4).

The following UV-source is generally used for applying the dose of morethan 50 mJ/cm²: a Fusion F600 W system having as lamps 1600M radiator(600 W/inch which equals 240 W/cm, and thus, in total 6000 W) fittedwith R500 reflector one, with a H bulb and one with a D bulb. For thepurposes of our invention, only the D-lamp is used to cure the samples.

A laboratory Macam lamp that is a 400 W metal halide lamp (Macam,Flexicure system) is used to pre-cure the film with short preflashes. UVlight is fed into the cell by a liquid filled light guide, resulting ina cut-off of the wavelengths below 260 nm (having a wavelength shorterthan 260 nm). Hence, it is preferred that the precure is performed witha first lamp having a different spectrum than the second lamp. On a drawtower this first lamp can be the lamp or lamps that cure the primarycoating before applying the secondary coating.

Thus, according to one embodiment, the present invention relates to amethod for curing a primary coating composition, said method comprisingthe steps of

(i) preparing a primary coating composition, which when cured withoutpreflash is having an equilibrium modulus of about 1.5 MPa or less and acavitation strength at which a tenth cavitation appears (σ¹⁰ _(cav)) ofat least about 0.9 MPa (preferably at least about 1.0 MPa) as measuredat a deformation rate of 0.20% min⁻¹, said cavitation strength beingabout 1.0 times or less (preferably, at least 1.1. times or less, morepreferred at least 1.2 times or less, most preferred at least 1.3 timesor less) of its storage modulus at 23° C. (E′₂₃), and(ii) curing said composition with a first dose comprising at least oneflash of UV-light of a total energy between about 5 and 50 mJ/cm² and(iii) subsequently curing the pre-cured coating with such a secondUV-dose that the pre-cured coating attains at least 85% of its maximumattainable equilibrium modulus, preferably, at least 90%, morepreferably, at least 95% of its maximum attainable equilibrium modulus.

Preferably, said first dose comprises at least one flash of UV-lighthaving a cut-off of the wavelengths below 260 nm, preferably below 250nm. The second UV-dose preferably does not contain a cut-off of thelower wavelengths, and thus, also contains wavelengths extending below260 nm, more preferred below 250 nm, most preferred below 240 nm.

The resistance to cavitation of the primary coating on a fiber can alsobe improved—independently of increasing the cavitation strength—bytuning the volumetric thermal expansion coefficient of the low modulusprimary coating of the present invention, and optionally, of both theprimary and secondary coating.

Therefore, another aspect of the present invention relates to a methodfor decreasing the volumetric thermal expansion coefficient of the lowmodulus primary coatings of the present invention, to a method fortuning the volumetric thermal expansion coefficients of both the primaryand secondary coatings that are used together as a system, and to suchimproved low modulus primary coatings as such.

The volumetric thermal expansion coefficient α₂₃ of a coating at 23° C.can be defined by the following formula (8):α₂₃=1/V(δV/δT)  (8)wherein V represents the specific volume (m³/kg) or the inverse of thedensity of the system, (δV/δT) represents the change in specific volumeof the system as a function of the temperature and T=23° C. In thepresent invention, α₂₃ is calculated by using the Synthia software ofMSI as explained further below.

It is accepted that a decrease in the volumetric expansion coefficientof the primary coating results in less shrinking of the primary when thetemperature is lowered and thus in less stress exerted on the primarycoating as confined between the glass substrate and secondary coating.

It is furthermore generally accepted in the fiber coating field that forpolymeric materials the thermal expansion coefficient and the Young'smodulus (called the “segment modulus” for primary coatings) areinterrelated, see E. Suhir, J. Lightwave Technology, 8, 863 (1990) andM. H. Aly, A. M. Shoaeb, M. Reyad, J. Opt. Commun. 2, 82 (1998). Often,the following linear relationship (9) is taken:

$\begin{matrix}{\alpha_{1} = {\alpha_{\bullet}\left( {1 - \frac{E_{1}}{E_{\bullet}}} \right)}} & (9)\end{matrix}$with:α₁, α•: the thermal expansion coefficient of the material of interest,respectively a reference material with low Young's modulusE₁, E•: the Young's modulus of the material of interest, respectively areference material with low Young's modulus

It was an object of the present invention to reduce the stress level inthe primary coating (and thus indirectly improve the cavitation strengthof the primary coating) and thus, to decrease the volumetric thermalexpansion coefficient α₂₃ of the primary coating while keeping itsmodulus low (and nearly constant).

The present inventors have now found that the Young's modulus andthermal expansion coefficient α₂₃ are not interrelated for the primarycoatings of the present invention. Since the storage modulus E′₂₃, whichis the modulus measured in a dynamic (DMTA) measurement, is nearlyidentical with the Young's modulus at 23° C., the same conclusion holdsfor the relation between α₂₃ and the storage modulus E′₂₃. The Young'smodulus and the storage modulus E′₂₃ are related to the network topologyof a primary coating system at 23° C., or alternatively the networkdensity of the coating system.

Furthermore, the thermal expansion coefficient for primary and secondarycoating systems is related to the cohesive energy density, defined asthe total amount of non-covalent interactions in the system, such ashydrogen bonding or dipolar interactions. Alternatively, one could saythat the volumetric expansion coefficient is related to the polarity ofthe system and not to the network density.

Therefore, according to one particular embodiment of the presentinvention, and in particular, for primary coatings having an equilibriummodulus of 1.5 MPa or less, the expansion coefficient α₂₃ of the primarycoating system can be decreased without having to increase the modulusof the primary coating system, preferably by increasing the cohesiveenergy density (CED) or the polarity of the system.

According to a preferred embodiment of the present invention, thecombined primary/secondary coating system comprises a primary coatinghaving a sufficiently low expansion coefficient α₂₃ and a secondarycoating system having a sufficiently high α₂₃ so that the stress levelin the primary coating is reduced to a level below the level of thecavitation strength of the primary coating. Preferably, the stress levelis below 0.8 MPa or below 1.2 times it's storage modulus at 23° C.(E′₂₃), more preferably below 0.5 MPa or below 0.9 times its E′₂₃, andmost preferred, the combination of primary and secondary coating ischosen such that the stress level in the primary coating isapproximately zero.

The thermal expansion coefficient α₂₃ for several coating systems can bepredicted on the basis of chemical structural information by usingcommercial software packages: the module Synthia of MSI (MolecularSimulations Inc, San Diego, Calif.) in combination with the Buildermodule of MSI. Synthia version 8.0 and the standard Builder modulewithin the Insight II (4.0.0P) graphical environment were used. Thecomputations were performed on a Silicon Graphics O2 workstation under aUnix based operating system. The builder module is applied for theconstruction of the chemical monomer species that will serve as inputfor the Synthia module. This module Synthia is based on a methodologydeveloped by J. Bicerano that is explained in detail in his monograph(J. Bicerano, Prediction of polymer properties, Marcel Dekker Inc., NewYork, 1993). This methodology makes use of compositional information,i.e. the chemical monomer structure, for the prediction of polymerproperties. In particular, connectivity indices based on graph theoryare used. This methodology is developed for the prediction ofproperties, among these properties the thermal expansion coefficient, oflinear amorphous homopolymers and for linear alternating and randomamorphous copolymers. The term linear refers to non-crosslinked systems.The primary coatings according to the present invention may be treatedas linear copolymers because their thermal expansion coefficient dependson the cohesive energy density, and thus most significantly on thepolarity and not on the network characteristics of the coatings. Thepolarity is identical for a network system or it's linear analogue. So,this linear analogue, a linear statistical copolymer is constructedbased on the chemical recipe of the coatings. The software programcalculates the thermal expansion coefficient at 23° C. (α₂₃).

According to one preferred embodiment of the present invention, theprimary coatings having an equilibrium modulus E of about 1.5 MPa orless have a volumetric expansion coefficient α₂₃ of about 6.85×10⁻⁴K⁻¹or less, preferably about 6.70×10⁻⁴ K⁻¹ or less, more preferred about6.60×10⁻⁴ K⁻¹ or less, even more preferred about 6.50×10⁻⁴ K⁻¹ or less,and most preferred about 6.30×10⁻⁴ K⁻¹ or less. Said primary coatingsshow an enhanced resistance to cavitation. Moreover, such primarycoating compositions surprisingly show an enhanced reactivity andphotosensitivity, and consequently a higher cure speed.

Preferably, the volumetric expansion coefficient of the secondarycoating α₂₃ of the present invention used in combination with theprimary coating of the present invention is at least about 3.15×10⁻⁴K⁻¹, preferably, at least about 3.20×10⁻⁴K⁻¹, more preferably at leastabout 3.30×10⁻⁴ K⁻¹, even more preferred about 3.50×10⁻⁴ K⁻¹, and mostpreferred at least about 4.0×10⁻⁴K⁻¹. The higher the α₂₃ of thesecondary coating used in combination with a primary coating, the lessstress is excerted on the primary coating.

On the other hand, it is desired that the α₂₃ of the secondary coatingis about 6.85×10⁻⁴K⁻¹ or less. Surprisingly, such a secondary coatingcomposition shows enhanced reactivity and photosensitivity, andconsequently a higher cure speed. Thus, by tuning the α₂₃ of thesecondary coating a desirable balance between an acceptable stress levelin the primary coating and an acceptable cure speed of the secondary canbe achieved. More preferably, α₂₃ of the secondary coating is about6.5×10⁻⁴ K⁻¹ or less, particularly preferred, about 6.2×10⁻⁴ K⁻¹ orless, more preferred about 6.0×10⁻⁴ K⁻¹ or less, and most preferredabout 5.8×10⁻⁴ K⁻¹ or less.

The relation between α₂₃ and rate (cure speed) is shown for thefollowing secondary-type coatings U, V, W and Z as shown in Table 1. Thepolarity decreases going from composition U to Z. Said coatings havebeen prepared with an identical concentration of double bonds, and witha same amount of di- or higher functional material, thus having the samecross link density. The rate has been measured by RT FTIR as describedin paragraph F of the description of test methods.

TABLE 1 relation between α₂₃ and cure speed of secondary-type coatingsU-Z Coating composition U V W Z Components Wt. % Wt. % Wt. % Wt. %HEA-IPDI-pTHF1000-IPDI-HEA 50 50 50 50 HEA 32.8 32.8 HEA-IPDI-5CC 17.2SR504 17.2 5.5 Butyl acrylate 31 Lauryl acrylate 19 Ethoxy ethylacrylate 44.5 Irgacure 184 1 1 1 1 Calculated α₂₃ (×10⁻⁴ K⁻¹) 6.42 6.717.04 7.35 Rate (mol/l sec) 2.99 2.67 2.50 2.29 Abbreviations andtradenames: HEA = 2-hydroxyethylacrylate; IPDI = isophoronediisocyanate; pTHF = polytetrahydrofuran having Mn of 1000; SR504 =ethoxylated (n = 4) nonyl phenol acrylate; Irgacure 184 = photo-initiator; HEA-IPDI-5CC = adduct of HEA, IPCI and

The results in Table 1 show that the cure speed increases upondecreasing volumetric expansion coefficient (and thus upon increasingpolarity) of the coating system.

Suitable coating compositions preferably contain one or more of thefollowing constituents: one or more reactive diluents selected from thegroup consisting of 1-(2-hydroxypropyl)3-phenoxy acrylate, vinylcaprolactam, vinyl pyrrolidone, N butylurethane O ethyl acrylate(CL1039), butyrolactone acrylate, acryloyloxy-dimethyl-butyrolactone,and the like, or mixtures thereof; one or more oligomers selected fromthe group consisting of polyether (urethane) acrylate, polyester(urethane) acrylate, polyether/polycarbonate copolymer based (urethane)acrylate, polyether/polyester copolymer based (urethane) acrylate andthe like, of which, an ethylene oxide/butylene oxide based urethaneacrylate and a polyether/polycarbonate copolymer based urethane acrylateare preferred.

The MSI Synthia software has also been used to calculate α₂₃ of theprimary coatings A, B and C, results are given in Tables 2 and 3.

Additionally, the coefficient of expansion of these UV-curable primarycoatings have been measured by TMA (Thermo mechanical analysis)measurements (see Table 2): a 200 μm (=L, thickness) layer was appliedbetween two quartz cups with a diameter of 9.5 mm, and cured with a doseof 1 J/cm². The heating rate was 2.5° C./minute, and measurements wereperformed from −60° C. to +80° C. From the ΔL/L (relative thicknesschange) the volumetric expansion coefficient α₂₃ at 23° C. wascalculated. The measurement was corrected for quartz expansion bysubstracting blanco curves.

TABLE 2 Volumetric expansion coefficients at 23° C. of primary coatingsα₂₃ (×10⁻⁴ K⁻¹) E′₂₃ α₂₃ (×10⁻⁴ K⁻¹) experimentally Coating (MPa)calculated by TMA A 0.4 7.89 8.0 B 1.1 7.01 7.0

The data show that the calculated and measured values for α₂₃ are wellin agreement.

TABLE 3 independency of α₂₃ of primary coatings B and C from the modulusCoating E′₂₃ (MPa) α₂₃ (×10⁻⁴ K⁻¹) calculated B 1.1 7.01 C 0.7 6.89

The data for primary coatings B and C show that even upon decrease ofthe modulus of the primary coating system, the α₂₃ can be remainedsubstantially unchanged, and hence, that while having a low modulus, theα₂₃ can remain low to reduce the stress on the primary coating.

TABLE 4 independency of α₂₃ of coatings D to F from the modulusOligomer/reactive diluent/PI E′₂₃ α₂₃ (×10⁻⁴ K⁻¹) Coating (wt %/wt %/wt%) (MPa) calculated D 28.5/68.5/3.0 0.62 7.33 E 48.5/48.5/3.0 1.43 7.12F 68.5/28.5/3.0 2.59 6.92

Coatings D to F each contain as the oligomer, an aliphaticpolyether-polycarbonate based urethane acrylate oligomer having anaverage Mw of 4000, as the reactive diluent, diethylene glycol ethylhexyl acrylate, and as the photoinitiator, Irgacure 184. The data forcoatings D, E and F show that even upon a major decrease of the modulus(from E′23=2.59 MPa for coating F to 0.62 MPa for coating D), the α₂₃only increases to a minor extent. This increase is primarily due to theslight change in polarity of the coating system going from coating F toD and is thus, independent of the modulus change.

In general, optical fibers are coated first with a primary coating andsubsequently with a secondary coating. The coatings can be applied as awet-on-wet system (without first curing of the primary) or as awet-on-dry system. The primary coating can be colored with a die, orsecondary coatings can be colored with pigments or dies, or a clearsecondary can be further coated with an ink. The primary and secondarycoatings generally have a thickness of about 30 μm each. An ink coatinggenerally has a thickness of about 5 μm (3-10 μm).

The coated and preferably colored optical fibers can be used in a ribboncomprising a plurality of said optical fibers, generally in a parallelarrangement. The plurality of optical fibers is further coated with oneor more matrix materials in order to obtain a ribbon. The presentinvention therefore further relates to a ribbon comprising a pluralityof coated and preferably colored optical fibers, generally in a parallelarrangement, said coated optical fiber comprising at least a primarycoating according to the present invention and preferably a secondarycoating according to the present invention.

The invention will be further elucidated by the following examples andtest methods.

Description of Test Methods

A. Measurement of Cavitation Strength

1. Measurement Set Up

The measurement set up consists of a digital tensile testing machineZWICK 1484 with digital control and with a video camera fitted on thetop (moving) plate of the machine (see Photograph 1). The sample is heldin place by a fixture connected to the load cell. The growth of thecavitations can then be followed in real time.

In order to obtain reproducible values of the cavitation strength, twomajor points should be kept in mind concerning the measurement set upitself. First, the parallellity of the set up is very important to allowa correct translation between the force at which cavitation starts andthe actual stress level in the cured coating. In the case of a goodparallellity of the plates, the stress distribution over the film willbe nearly flat, the coating layer is then (approximately) subjected to ahomogeneous triaxial stress level σ, equal to the ratio (force/samplearea).

If the alignment of the set up is imperfect, however, the sample mayexperience a torque resulting in an inhomogeneous tearing of the polymerfilm. In this case, the inhomogeneous stress distribution makes itdifficult to translate the registered force signal into the actualstress in the film.

The parallelity can be finely adjusted using three micrometer screws onthe moving plate of the tensile testing machine (Photograph 2). Circularglass plates (40 mm in diameter, at least 5 mm in thickness) were used(FIG. 2) and by using three hardened steel balls fitted to theadjustable plate one can—within the accuracy of the micrometer screws,about 1 μm—adjust the parallelity of the sample in the measurement setup.

Another important factor is the stiffness of the entire set up: thecompliance of the measurement set up should be as low as possible toavoid any storage of elastic energy in the measurement set up. Theadjustable plate was therefore made of 15 mm thick steel resulting in acompliance of approximately 0.2 μm/N for the total set up. Thecompliance is measured by using a welded steel sample having the samegeometry as in FIG. 2 and is determined from the measured force anddisplacement.

2. Sample Preparation

All glass plates (boro silicatum glass) and quartz billets were finelypolished using carborundum powder to such an extent that the roughness(Ra) of the glass plates has a value of 1.17±0.18 μm and the roughness(Ra) of the quartz billets has a value of 1.18±0.04 μm. Subsequently,the glass and quartz pieces were burned clean in an oven at 600° C. forone hour and the surfaces were rinsed with acetone and allowed to dry.The clean surfaces were kept in a closed container to avoid dustsettling.

The surfaces were treated with a silane solution as follows:

A Preparation of Silane Solution

A 95% ethanol-5% water solution was adjusted to pH 4.5-5.5 with aceticacid, and a silane (Methacryloxypropyltrimethoxysilane, A174 from Witco)was added to yield a 5% silane solution (ca. 74.39% wt ethanol/3.84% wtwater/16.44% wt acetic acid/5.32% wt silane). The silane solution wasleft for five minutes at room temperature to allow hydrolysis andsilanol formation. The fresh silane solution was applied to the glass orquartz surfaces by using pipette. The silane layer was cured by placingthe treated glass or quartz plates in an oven at 90° C. for five to tenminutes. The treated glass or quartz plates were rinsed free of excessmaterials by gently dipping in ethanol.

The example was assembled as follows:

A quartz cup was attached to the top plate of the two-plate micrometerusing a vacuum system (vacuum pump) (Photograph 4).

The micrometer was zeroed using both the quartz billet and the glassplate. A droplet of resin was gently placed in the middle of the glassplate.

The glass plate was placed on the lower plate of the two-platemicrometer and the film thickness was adjusted by slowly pushing thequartz billet onto the resin droplet. Subsequently, the sample was curedwith a 1 J/cm² UV-dose of Fusion F600 W UV-lamp system having as lamps1600M radiator (600 W/inch which equals 240 W/cm, and thus, in total6000 W) fitted with R500 reflector, one with a H bulb and one with a Dbulb UV lamp, of which the D-bulb was used to cure the samples.

The samples were stored in a dark place, so that no post-cure byUV-light can take place.

Cured samples were measured within 1-2 hours after preparation.

3. Measurement

The sample was placed in the tensile testing apparatus from ZWICK type1484.

When an experiment was started, a video camera recorded the behavior ofthe film while showing the stress exerted on the film. Unless otherwisestated, the pulling speed was 20 μm/min. The microscope was used toachieve about 20× magnification (the 9.5 mm sample was enlarged to 19 cmat the video screen). From the videotape, the appearing of a number ofcavities at a certain measured stress was noted.

Sample Preparation for all Measurements Under Paragraphs B-E

The samples were cured with a 1 J/cm² UV-dose of a Fusion F600 W UV-lampsystem (measured with AN international Light 390-bug) as described underparagraph A above using a D bulb at a belt speed of 20.1 m/min.

B. Measurement of Equilibrium Modulus, Storage Modulus at 23° C. (E′23)and Glass Transition Temperature in DMTA

The equilibrium modulus of the primary coatings of the present inventionis measured by DMTA in tension according to the standard Norm ASTMD5026-95a “Standard Test Method for Measuring the Dynamic MechanicalProperties of Plastics in Tension” under the following conditions whichare adapted for the coatings of the present invention.

A temperature sweep measurement is carried out under the following testconditions:

Test pieces: Rectangular strips Length between grips: 18-22 mm Width: 4mm Thickness: about 90 μm Equipment: Tests were performed on a DMTAmachine from Rheometrics type RSA2 (Rheometrics Solids Analyser II)Frequency: 1 Hz Initial strain: 0.15% Temperature range: starting from−130° C. heating until 250° C. Ramp speed: 5° C./min Autotension: StaticForce Tracking Dynamic Force Initial static Force: 0.9 N Static >Dynamic Force 10% Autostrain: Max. Applied Strain: 2% Min. AllowedForce: 0.05 N Max. Allowed Force: 1.4 N Strain adjustment: 10% (ofcurrent strain) Dimensions test piece: Thickness: measured with anelectronic Heidenhain thickness measuring device type MT 30B with aresolution of 1 μm. Width: measured with a MITUTOYO microscope with aresolution of 1 μm.All the equipment is calibrated in accordance with ISO 9001.

In a DMTA measurement, which is a dynamic measurement, the followingmoduli are measured: the storage modulus E′, the loss modulus E″, andthe dynamic modulus E* according to the following relationE*=(E′²+E″²)^(1/2).

The lowest value of the storage modulus E′ in the DMTA curve in thetemperature range between 10 and 100° C. measured at a frequency of 1 Hzunder the conditions as described in detail above is taken as theequilibrium modulus of the coating. The storage modulus E′ at 23° C. inthe DMTA curve is taken as E′23.

C. Measurement of Stress/Strain Curves, and Description of RelativeMooney Plots

A relative Mooney plot can be obtained as follows:

C.1. Stress/Strain Curve from an Uniaxial Tensile Test

The primary measurement is the force-displacement curve, measuredaccording to International Standard ISO 37 (third edition 1994-05-15)

“Rubber, vulcanised or thermoplastic—Determination of tensilestress-strain properties” which is an uniaxial tensile test. Thefollowing conditions are applied for the primary coatings of the presentinvention:

Test pieces: Dumb-bell piece: type 3 Initial length lo: about 18-20 mmThickness: about 90 μm Equipment: Tests were performed on a tensilemachine from ZWICK type 1484 Force cell: 50 N Elongation: Measured withan optical device with a resolution of 0.002 mm/measured increment Typeof clamps: Keilchraub-Problemhalter 8106.00.00 F_(max) = 500 N Testspeed: between 0.1-500 mm/min, depending on the property measuredDimensions dumb-bell: Thickness: measured with an electronic Heidenhainthickness measuring device type MT 30B with a resolution of 1 μm Width:measured with a MITUTOYO microscope with a resolution of 1 μm Testtemperature: 23° C. ± 2° C. at 50% RH ± 10% Number of specimen: Between3 and 5All the equipment is calibrated in accordance with ISO 9001.C.2. Relative Mooney Plot

The above force-displacement curve is measured using a clampdisplacement speed of 5 mm/min, 50 mm/min or 500 mm/min. Preferably at500 mm/min because at higher speed, the effect of the material (strainhardening effect) can be visualized better, in particular in case ofmaterials for which the strain hardening sets in only at higher strain.From this measurement the engineering stress is calculated according toformulas (1)-(6) as given in the description.

D. Measurement of Strain Energy Release Rate or Tear Strength (Go)

The strain energy release rate Go is measured according to theInternational Standard norm ISO 816 (second edition 1983-12-01) “Rubber,vulcanized—Determination of tear strength of small test pieces (Delfttest pieces)” under the following specific conditions:

Test pieces: in accordance with the ISO 816 Length between 50 mm grips:Thickness (d): about 90 μm Slit length (b): length of initial crack isdefined in ISO 816 Equipment: Tests were performed on a digital tensilemachine from ZWICK type 1484 Type of clamps: Keilchraub-Probenhalter8106.00.00 F_(max) = 500 N Force cell: 50 N Elongation: Measured withmachine displacement with a resolution of 0.01 mm/measured incrementTest speed: 0.1 mm/min Dimensions test Thickness (d): measured with anelectronic piece: Heidenhain thickness measuring device type MT 30B witha resolution of 1 μm Width (B): measured with a MITUTOYO microscope witha resolution of 1 μm Test temperature: 23° C. ± 2° C. at 50% RH ± 10%Number of Between 3 and 5 specimen:All the equipment is calibrated in accordance with ISO 9001.

The strain energy release rate Go is the energy required per 1 m² crackin the above described test specimen of a cured primary coatinginitially containing a small crack equal to slit length b as defined inISO 816. Go is then calculated as follows:

$\begin{matrix}{{Go} = \frac{\left( {\frac{Fbreak}{B{\cdot d}} \cdot C \cdot \sqrt{\pi\;\frac{b}{2}}} \right)^{2}}{E}} & (10)\end{matrix}$wherein F_(break) is the force at break, b is the slit length, d is thethickness and B the width of test piece, and E is the segment modulus attest speed of 0.1 mm/min as calculated from the stresses at elongationof 0.05 and 2% in test method as described in paragraph C.1 and whereinC defines the sample geometry as follows:

$\begin{matrix}{C = \sqrt{\frac{1}{\cos\;\frac{\pi\; b}{2B}}}} & (11)\end{matrix}$E. Measurement of Adhesion Testing

The adhesion of cured samples on a glass plate at 50% relative humidityand 95% relative humidity were tested using a universal testinginstrument, INSTRON Model TTD. The load cell had a ten pound (3732 gram)capacity. Glass plates, polished, 20×20 cm, (Alletch Associates catalognumber 26080) were used. The test material was applied to the glassplates and cured at 1 J/cm² with a Fusion D Lamp under Nitrogenatmosphere. The thickness of the cured film was about 75 microns.

The cured films were held at 50% relative humidity, at about 23° C., forseven days prior to testing.

Test specimens, approximately 25.4 mm in width and 127 mm long, were cutparallel to the direction in which the drawdown of the cured film wasprepared. A thin layer of talc was applied to the first and third stripson each drawdown to reduce blocking during the adhesion test.

The instrument was calibrated prior to testing. The crosshead speed wasset to 254 mm/min. For each material, the force required to remove fourtest specimens from the glass plate was measured and recorded on a stripchart recorder. The value reported is the average of the four measuredvalues. The test specimens remaining on the glass plate were then heldat 95% relative humidity, at about 23° C., in an environmental chamberfor 1 more day. Prior to removing the plates from the environmentalchamber, a layer of slurry (fine powdered polyethylene and water) wasapplied to the surface of the drawdown to retain the moisture. For eachmaterial, the force to remove four test specimens from the glass platewas measured as above.

F. RT FTIR Measurements

A 10 micron thick layer of the reactive composition on a gold coatedAlumina plate was cured in a RT-FTIR instrument under a nitrogenatmosphere (Bruker IFS 55 equipped with a transflection cell and a UVsource, an Oriel system with a 200 W Hg lamp, for a full description ofthe equipment see: A. A. Dias, H. Hartwig, J. F. G. A. Jansen conferenceproceedings PRA Radcure coating and inks; curing and performance June1998 paper 15). The consumption of acrylate bonds was measured at 21° C.during the curing by this technique and the maximum rates of acrylateconversions (in mol/l sec) were calculated according to the above citedreference.

EXAMPLES AND COMPARATIVE EXPERIMENTS Comparative Experiment A

A coating was prepared using 50 wt % of a polyether urethane acrylate(theoretical molecular weight≈9000), about 20 wt % of 8 timesethoxylated nonylphenol acrylate, 20 wt % laurylacrylate, 6 wt % ofN-vinylcaprolactam, 1.5 wt % Lucirine TPO, 0.8 wt % Irganox 1035, 0.1 wt% diethanolamine and 0.3% Seesorb 102.

Results of tests are as follows: equilibrium modulus 0.4 MPa, E′23 is0.5 MPa, very little strain hardening, Go=26 J/m². Curing was achievedwith 1 J/cm² irradiation. In the relative Mooney plot of FIG. 4, thiscoating is represented by the curve for Comp. Exp. A. The cavitationstrength at the 10^(th) cavitation σ₁₀ was measured to be 0.96 MPa (seeFIG. 3).

Comparative Experiment B and Example 1

A commercial coating with an equilibrium modulus of 1.0 MPa was used. Incomparative experiment B, the sample was cured using a dose of 1 J/cm².The cavitation strength measured at the occurrence of the cavitation was1.21 MPa. The storage modulus E′ at 23° C. of the coating E′23 is 0.97MPa. Hence, the value of σ₁₀/E′23 was 1.25, Go=22 J/m².

In Example 1, the sample was first cured with three short flashes ofUV-light (in total about 1 cJ/cm² or less), whereafter the sample wascured at 1 J/cm² (as described in the Description of Test Methodsabove). A laboratory Macam lamp which is a 400 W metal halide lamp(Macam, Flexicure system) is used to pre-cure the sample (the threeshort flashes of 1 cJ/cm² or less in total): UV-light is fed into thecell by a liquid filled light guide, resulting in a cut-off of thewavelengths below 260 nm.

The cavitation strength of the coating of Example 1 measured at thetenth cavity σ₁₀ was 1.47 MPa and E′23 was 0.97 MPa (see FIG. 3), hence,the σ₁₀/E′₂₃ was 1.52.

FIG. 3 shows the number of cavitations observed at increasing stresseson the primary coating sample without (Comp. Experiment B) and with(Ex. 1) precure.

Example 2

A coating was formulated using 69.7 wt % of a polyether urethaneacrylate oligomer having a polyether backbone comprising on average twoblocks polypropylene glycol having an average Mw of about 4000 and beingend-capped with ethoxy groups (the oligomer is the reaction product of apolyether polyol, toluene diisocyanate and 2-hydroxyethyl acrylate),20.4 wt % 2-phenoxyethyl acrylate, 6.4 wt %tripropyleneglycoldiacrylate, 2.0 wt. % Lucerin TPO, 0.3 wt % DC190, 0.2wt % DC57 and 1 wt % mercapto silane.

The equilibrium modulus was 0.6 MPa. The modulus E′ at 23° C. was 0.7MPa. The relative Mooney plot is given in FIG. 4. The cavitationstrength at the tenth cavity σ₁₀ was measured to be 1.24 MPa after 1J/cm² cure (see FIG. 3); the σ₁₀/E′₂₃ was 1.77; Go=31 J/m².

Example 3

A primary coating composition was formulated using 38.8 wt % of analiphatic polyether-polycarbonate based urethane acrylate oligomerhaving an average Mw of 4000 (the oligomer is derived from2-hydroxyethylacrylate, isophoronediisocyanate, and equal amounts ofpolypropyleneglycol diol and a copolymer diol of 10-15 wt %polyether/85-90 wt % polycarbonate), 48.5 wt % of N butylurethane Oethyl acrylate (CL1039), 9.7 wt. % isodecyl acrylate and 3 wt % ofIrgacure 184 photoinitiator.

The equilibrium modulus is 1.31 MPa. The calculated volumetric thermalexpansion coefficient α₂₃ is 6.74×10⁻⁴ K⁻¹.

Comparative Experiment C

A coating was prepared using 60 wt % of a polyether urethane acrylate(theoretical molecular weight≈4000), 18.6 wt % of 4 times ethoxylatednonylphenol acrylate, 4 wt % 1-(2-hydroxypropylphenoxy acrylate, 7 wt %laurylacrylate, 7.8 wt % of N-vinylcaprolactam, 1.2 wt % Lucirine TPO,0.3 wt % Irganox 1035, and 0.1 wt % diethanolamine.

Results of tests are as follows: equilibrium modulus 1.2 MPa. Thecalculated volumetric thermal expansion coefficient α₂₃ is 7.15×10⁻⁴K⁻¹.

The comparison between the primary coatings of Example 3 and ComparativeExperiment C shows that the actual stress level in the coating ofExample 3 (with an α₂₃ within the claimed range) is lowered compared tothe stress level in the primary coating of Comparative Experiment C. Thegain on the lowering of the stress in the primary coating is dependingon de thermal volumetric expansion coefficient of the secondary coating.Typically, for a secondary coating having an α₂₃ of 3.0×10⁻⁴ K⁻¹, thestress in the primary of Example 3 is about 20% lower than the stress inthe primary coating of Comparative Experiment C. For a secondary havingan α₂₃ of 3.5×10⁻⁴ K⁻¹, the stress in the primary of Example 3 is about50% lower than the stress in the primary coating of ComparativeExperiment C. Thus, the resistance to cavitation is increased going fromprimary coating of Comparative Experiment C to primary coating ofExample 3. Furthermore, this comparison shows that the stress in theprimary coating further lowers upon increasing α₂₃ of the secondarycoating which is used together with the primary coating from 3.0×10⁻⁴K⁻¹ to 3.5×10⁻⁴ K⁻¹.

Example 4

A secondary coating composition was formulated using 50 wt. % of analiphatic polyether based urethane acrylate oligomer (the oligomer isderived from 2-hydroxyethylacrylate, isophorone diisocyanate, andpolytetrahydrofuran having Mn of 1000), 32.8 wt. % of2-hydroxyethylacrylate, 17.2 wt. % of HEA-IPDI-5CC adduct (as describedfor coating U in Table 1 above) and 1 wt. % of Irgacure 184.

The calculated α₂₃ is 6.42×10⁻⁴ K⁻¹, the rate of polymerization is 2.99mol/l sec.

Example 5

A secondary coating composition was formulated using 50 wt. % of analiphatic polyether based urethane acrylate oligomer (the oligomer isderived from 2-hydroxyethylacrylate, isophorone diisocyanate, andpolytetrahydrofuran having Mn of 1000), 32.8 wt. % of2-hydroxyethylacrylate, 17.2 wt. % of ethoxylated (n=4) nonyl phenolacrylate and 1 wt. % of Irgacure 184.

The calculated α₂₃ is 6.71×10⁻⁴ K⁻¹. The rate of polymerization is 2.67mol/l sec.

1. An assembly for measuring the cavitation strength of a coatingcomprising: a first member having a surface; a second member having asecond surface opposing said first surface; at least one of said firstmember and said second member being transparent to ultraviolet light;said first surface being moveable in a direction normal towards saidsecond surface; said first surface defining with said second surface acavity for receiving a sample; and a sub-assembly in contact with saidfirst member or said second member; said sub-assembly comprising atleast one element capable of adjusting the position of said firstsurface or said second surface in such a manner that said first surfaceor said second surface is perpendicular to the direction of said normalmovement.
 2. An assembly according to claim 1, wherein both said firstsurface and said second surface are perpendicular to the direction ofsaid normal movement.
 3. A tensile testing apparatus comprising theassembly according to claim
 1. 4. Method for measuring the cavitationstrength of a radiation cured coating comprising the steps of: making asample by treating two plates by applying a liquid coating in betweenthe two plates in a thickness of between 10 and 300 μm and over acertain area and by curing said coating with a UV-dose, the treatment ofthe two plates being such that the adhesion between the plates and thecured coating is sufficient to obtain cavitation before debonding setsin, placing the sample in a tensile testing apparatus, which is providedwith a microscope, in such a way that a substantially parallel alignmentand an acceptable compliance of the total tensile testing apparatus isobtained, running a deformation test on said sample while measuring theforce at which a defined number of cavities starts to be visible throughthe microscope at a certain magnification, and calculating the stress bydiving said force by the area of the coating applied and reporting saidstress in relation to said cavities.